Preheat the oven to 350°F (175°C).
Prepare the Chicken flavor Rice-A-Roni according to the package directions in a skillet or saucepan; cook until the rice is tender and liquid is absorbed.
In a mixing bowl, combine the prepared Rice-A-Roni with the can of unsalted cream of chicken soup, sour cream, and the cooked chopped chicken; stir until evenly combined.
Spread the rice and chicken mixture into a lightly sprayed or greased 9×13-inch baking dish and smooth the top.
Sprinkle the French fried onions evenly over the top of the casserole.
Bake the casserole uncovered in the preheated oven for 30 minutes, until heated through and the topping is golden.